largest carb for a chinese 80cc bike kit

Hello. all
I was wondering what's the largest size carb anyone knows of being installed on these bike kits. I just finnished installing a 42mm Keihin carb on mine and seems to run as well as it did with the stock carb. now it's time to try tune it for top speed. I trilled out the main jet in my stock carb to 1.5mm lol guess what pretty much had to hold a truck box till i was up to speed to match the fuel.. but anyway i figured if i want more fuel ill just install a larger carb. anyway point is will this tune to run right or wasting time.?

it's not just the engine size that counts but also the max RPM.
20mm is the biggest useful size for your size engine maxing out at 7000 rpm.
anything more than that offers no advantages.
but the Keihin is a fully adjustable carb, unlike the crummy stock carb.
have fun!

cool. i had this one already. thought i would give it a go. so much better than the old one. at 1/4 throttle 2700rpm i now cruise at my old top speed @ 5600rpm. i thought i was having jetting issues as it only seemed to be lacking at the top end. i have expansion chamber from a pocket bike modded to fit. but have not done any porting yet. should I? I am currently running rich it seems as its very boggy. what im wondering is while im cruising alone and it's bogging every once in a while it will start to sound like a 2 stroke should and accelerate some but then back to the bogging. it's weird cause to me it acts like it wants more fuel.

if i take a mm of the head would that be to much. is there a rough average as to how much compression 1mm would add. ? I seen another post with the fella that has a very large expansion chamber installed. love that.. looking at some of my sleds to see what one is going to donat a pipe.. lol im just working on uploading a video. i will post soon. maby you can recommend some things..

if i take a mm of the head would that be to much. is there a rough average as to how much compression 1mm would add. ? I seen another post with the fella that has a very large expansion chamber installed. love that.. looking at some of my sleds to see what one is going to donat a pipe.. lol im just working on uploading a video. i will post soon. maby you can recommend some things..

Well compression is now 136psi. now retuning the carb, had alot more smoke now that i did not before. infact as rich as i ran before i never seen some come from both tips of my exhaust.. i opened up the intake gasket somemore to better match the head it's no longer blocking the intake.. i am still getting alittle air in through intake tube and carb working on a way to seal it up. I think my exspansion chamber is still a restriction as it's from a 49cc pocket bike and is on the small size maby 2" at its largest point. and looks like about 3/4" tubing..

Your expansion chamber could very well be doing more harm than good.
They need to be timed so the shock wave pulse hits the clean new fuel mix coming into the exhaust and shove it back in the cylinder just before the port closes.

if i take a mm of the head would that be to much. is there a rough average as to how much compression 1mm would add. ? I seen another post with the fella that has a very large expansion chamber installed. love that.. looking at some of my sleds to see what one is going to donat a pipe.. lol im just working on uploading a video. i will post soon. maby you can recommend some things..

Click to expand...

A mm is to much for the slant plug head. In my experimenting I found by adding a second head gasket of .027 compression dropped 30 lbs.. So that's about 1.1 lb. per .001. I remove the material from the cylinder, and with a slant plug head I make the cylinders 2.730 top to bottom.

there's too much variance between different engines so you first need to get a pressure gauge to see what it has now.
I think slant plug heads already cause enough compression on most engines.
If your setup has under 130psi then just remove a little metal at a time and keep rechecking the pressure.